The demand for high bandwidth wireless communications is unrelenting and, as such, the requirements placed upon cellular networks are always increasing. In particular with the onset of wireless multimedia communications it is desirable to be able to offer high data rate communications in both directions of a cellular communication system, i.e. to and from a wireless subscriber communication unit.
However, as radio spectrum is such a precious resource, it is typical that radio frequencies used in one cell (coverage area) may be used simultaneously by the adjacent cells (typically other overlapping coverage areas) as well. In addition the resources used within one cell may be simultaneously shared amongst several users connected to that cell. Thus, in a cellular network there may be several simultaneous communications occurring on the same frequency, or same set of frequencies, as well as the same time instances.
At a receiving communication unit in such a communications network, be it a wireless subscriber communication unit or a base station, ‘composite’ signals arrive that correspond to the desired communication signal and also that represent several possible simultaneous interfering communication signals occurring on the same frequency. These latter communications represent interference at the receiving communication unit, the effects of which need to be mitigated in order to successfully demodulate the desired communication signal.
Typically, the data rate that is sustainable in a communication link is proportional to a received level of the desired signal above a level of any interfering signal(s) and noise, referred to as the Signal to Interference plus Noise Ratio (SINR) or sometimes shortened to just SNR. Thus, a higher data rate is generally more achievable at a high SINR (or SNR) than at a low SINR (or SNR).
In a congested cellular environment with many wireless subscriber communication units requiring simultaneous communication links, the interfering signals tend to dominate the background noise and, thus, the interference is the aspect that dictates the achievable communication data rate. This is referred to as an interference limited environment. It is thus clear that if the interference could be removed then a higher SNR could potentially be achievable with a commensurate increase in the communication data rate.
The interfering signals typically originate from three possible sources, namely:    other simultaneous communication links from within the same cell of the same communication system, this is generally referred to as intra-cell interference;    simultaneous communications links from other cells of the same communication system, this is generally referred to as inter-cell interference;    simultaneous communication links from other communications systems, such as leakage from communications systems operating in adjacent frequencies, other communications systems operating in the same frequency spectrum, i.e. unlicensed spectrum, etc.
Most advanced cellular communications systems have been designed in order that either the intra-cell interference is avoided or can be readily removed at the receiving device. For instance in TD-CDMA the intra-cell interference is detected and, hence, removed as part of the receiver processing using a multi-user detector (MUD), whilst in OFDM intra-cell interference is typically avoided by using orthogonal tones for different simultaneous users within the same cell.
Interference due to simultaneous communication links from other communication systems is somewhat more difficult to remove. In principal this may be performed as part of receiver processing using additional signal processing techniques.
Inter-cell interference is difficult to avoid on a network-wide scale, in that it would require a scheduler to allocate orthogonal resources to simultaneous users across many cells within the network. Thus, one method for removal of inter-cell interference is to allow the receiving communication units in the network to detect signals not only from the sources in the same cell, but also simultaneously from other cells within the communications network. This could be described as an advanced or inter-cell capable multi-user detector (MUD), a description of which can be found in GB412036.
The aforementioned prior art refers to the case when all the communications signals are synchronous or approximately synchronous (sometimes referred to as being ‘block’ synchronous). In the context of this patent specification, synchronous or block synchronous may be defined as encompassing a case where the separate communications signals are received within a certain predefined window. This predefined window will typically be a small percentage of the timeslot, burst etc. For instance, in the TD-CDMA case, this window will typically be equivalent to the channel estimation window, whilst in the OFDM case this window will be typically equivalent to the cyclic prefix duration.
However, the detection of inter-cell interference becomes significantly more complicated when the signals arrive at the receiving device in a substantially asynchronous manner. This may arise due to either the cells in the network being unsynchronised or the distance between cells being large enough such that propagation times of the various communications propagation paths are substantially different.
This latter case is illustrated in FIG. 1 for the downlink scenario. In FIG. 1 time-synchronised cellular base stations 105, 110 are respectively transmitting a burst 120, 125, which is received at a wireless subscriber communication unit 115, where a burst is a generic term often used to include a communication frame, slot, burst, sub-frame, timeslot, block of data, etc. As illustrated, the receiving communication unit is located substantially closer to cell A's transmitter 110 than cell B's transmitter 105. Thus, although the bursts 120, 125 were transmitted simultaneously, when they are received at the receiving communication unit 115 the bursts appear highly ‘asynchronous’ as can be seen from the inset timing diagram 135. The delay 130 is proportional to the difference in the propagation path distances from the respective base stations 105, 110 to the receiving communication unit 115. It should be noted that although the illustration is for the downlink scenario (communication from a base station to a receiving subscriber communication unit) it equally applies to the uplink (communication from a transmitting subscriber communication unit to a base station), i.e. the base stations and subscriber communication units are interchanged in the illustration.
To optimally detect asynchronous interfering signals at a receiving device the signal processing within the receiver, and/or the structure of the communicated signal, must be designed to cope with a maximum length of the expected delays 130. To highlight this we refer to the examples of TD-CDMA and OFDM mentioned previously.
In TD-CDMA, the most common method of signal detection at the receiving device is to employ a linear MUD. Such an algorithm essentially performs a matrix operation on a vector of received signals to separate the signal into its constituent signal components. In the most common realisations of the linear MUD the key step is the inversion of a system matrix, which describes the structure of the signals of the various simultaneous communications links and the propagation channels that these signals have been subjected to. This can be extended to include inter-cell communications signals as described in GB412036. With such methods the complexity of the detection operation is directly linked to duration of the propagation channels assumed in the system matrix, with a longer duration propagation channel leading to considerable additional complexity in the detection operation.
Thus, optimal detection of inter-cell TD-CDMA signals would require the channel window of the MUD to be of sufficient duration to encompass the expected asynchronicity of the desired signal component and the inter-cell communication signals, illustrated in FIG. 1. If the cellular communication system is asynchronous, or the communication signals are expected to traverse large distances, then this channel window quickly becomes unacceptably large and leads to practically unrealisable detection algorithms in the receiving communication unit. Thus, in practice, a MUD would be designed with a channel window that is acceptable from an implementation perspective and then the level of asynchronicity tolerated within the receiver architecture would be governed by this channel window, to enable signals arriving within the channel window to be detectable. Thus, signals with a delay, which are greater than the implemented channel window, would remain undetected.
In 3GPP High Chip rate TD-CDMA, the burst structure is typically designed to allow a channel window of 57-64 chips, which represents a small percentage of the total burst duration of 2560 chips. Typically, a MUD for this application would then be designed to accommodate this channel window duration. Thus, it is clear that only a limited amount of asynchronicity could be tolerated in such a system before inter-cell communication signals become undetectable and, hence, the inter-cell interference ceases to be mitigated.
An OFDM system employs bursts or symbols that typically comprise a data portion and a cyclic prefix, or suffix. For simplicity, within the hereinafter described implementation, we refer to only the case of a cyclic prefix. However, a skilled artisan will appreciate that the inventive concept may be applied to either a cyclic prefix or cyclic suffix. The cyclic prefix is analogous to the channel window in TD-CDMA as described above. That is to say that the cyclic prefix is provided to allow for any multipath delays experienced by the communication signal as it travels through the propagation environment. The cyclic prefix is formed by copying the last few samples of the data portion of the burst and appending these to the front of the data portion, thus making a cyclically symmetric burst or symbol. In an OFDM receiver, the data portion is extracted from the cyclic prefix and converted to the frequency domain where the individual tones are orthogonal. If a tone constitutes communication signals from multiple cells these may be optimally separated.
However, if the data extraction of the OFDM burst is not aligned correctly, and starts earlier than the start of the cyclic prefix or later than the end of the cyclic prefix, the extracted portion will no longer be cyclically symmetric. Thus, once converted to the frequency domain the individual tones are not orthogonal and optimal detection of the various communication signals is no longer a feasible operation.
Thus, in a similar manner to TD-CDMA, it is clear that only a limited amount of asynchronicity can be tolerated before inter-cell communication signals become undetectable in an OFDM implementation and, hence, the inter-cell interference ceases to be mitigated by conventional means.
The above examples serve to demonstrate the failings of the prior art when the interfering signals are substantially asynchronous. This situation represents an unsynchronised cellular communication system, or one with considerable distances between each of the cells or in fact one where another communication link is not necessarily part of the cellular communication system.